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Mechanical ventilatory assistance is now widely accepted as an effective form of therapy and means for monitoring respiratory failure in the neonate. Mechanical ventilators are a conspicuous and fundamental part of neonatal care. When on assisted ventilation, the newborn infant becomes part of a complex interactive system which is expected to provide adequate ventilation and gas exchange.
The overall performance of the assisted ventilatory system is determined by both physiological and mechanical factors. The physiological determinants, over which the physician has relatively little control, change with time and are difficult to define. These include the nature of any pulmonary disease, the ventilatory efforts of the infant, and many other anatomical and physiological variables. Mechanical input to the system, on the other hand, is to a large extent controlled and can be reasonably well characterized by examining the parameters of a ventilator pressure pulse. Optimal ventilatory assistance requires a balance between physiological and mechanical ventilation. This balance should insure that the infant is neither overstressed nor oversupported. Insufficient ventilatory support would place unnecessary demands on the infant's compromised respiratory system. Excessive ventilation places the infant at risk for pulmonary barotrauma and other complications of mechanical ventilation.
Intelligent management of ventilatory assistance in the neonate requires that information about the performance of the overall system be available to the clinician. Instruments for continuous monitoring of infants on assisted ventilation, as well as certain component variables of ventilation, are known and are discussed in "Instruments for the Continuous Measurement of Gas Exchange and Ventilation of Infants During Assisted Ventilation", K. Schulze, M. Stefanski, J. Masterson, L. S. James, Critical Care Medicine, Vol 11, No. 11, pp. 892-896 (1983), incorporated herein by reference. However, at the present time, physicians rely largely on intermittent measurement of arterial blood gases to monitor the overall effects of the system on gas exchange. These measurements, while important in clinical care, have several limitations. Data acquired by such measurements provides little information about the separate contributions of the infant and the mechanical ventilator to overall ventilation and gas exchange of the infant. It has also been recognized that mechanical ventilation, although potentially a very promising technique, may indeed be harmful to the lungs and brain of the infant in the event that the mechanical ventilator is not properly synchronized with the infant's breathing. For example, see A. Greenough, C. Morley, J. A. Davis, "Interaction of Spontaneous Respiration with Artificial Ventilation in Pre Term Babies", Journal of Pediatrics 103:769 (1983). Furthermore, difficulty has been encountered in calibrating known systems to provide accurate and precise measurements of flow rates and the like.
Absent information concerning the respective contributions of ventilation, the effects of changes in ventilator support are not as readily observable. For example, it is frequently desirable to monitor how an infant responds to respiratory therapy such as positive end expiratory pressure ("PEEP") therapy. To administer this therapy, the ventilator decreases resistance to expiratory gases, thus decreasing the burden on an infant's lungs.
In addition, arterial blood gas measurements are available only intermittently in known systems. Unfortunately, this makes both trends and abrupt changes in clinical condition of the patient difficult to recognize. Continuous values are appreciably more helpful in describing the time course of changes in the patient's clinical condition than instantaneous or intermittent values.
When acquiring measurements of infant ventilation for research purposes, it is customary to place the infant in a container known as a plethysmograph. A plethysmograph is a standard device for measuring change in volume of any mass contained within it. Because changes in volume of animals or humans are due entirely to the flow of gas into and out of the lungs this device affords an elegant approach for the measurement of pulmonary function.
Normally, a plethysmograph is configured as an airtight box enclosing a subject who breathes externally supplied gas directly through an endotracheal tube inserted into the subject's nose, through his throat and into his windpipe. Thus, any gas breathed by the subject must be suppled from and exit via appropriate portions of the endotracheal tube and related piping. This gas breathed by the subject is supplied from a gas source and does not communicate with air contained within the plethysmograph. As the subject inspires, the volume of the chest increases and causes either the pressure to rise inside the plethysmograph chamber if it is closed or, if the plethysmograph chamber has an opening, air to flow out, or a combination thereof. These changes in pressure or flow can be measured using any of a variety of sensors. With the exception of openings used for respiratory support of the infant, and quantitative measurement of the infant's respiration, the interior of the plethysmograph must be isolated from the external environment. Also, for these quantitative measurements to be useful in patient care, it is desirable to configure the plethysmograph such that the sensors are in a relatively stable environment. At the same time, however, it is essential that the infant remain warm and undisturbed and also be accessible in a very short period in the event that an emergency arises.
Unfortunately, accuracy of measurements made by known plethysmographs is severely compromised by the inherent compliance of air contained within the plethysmograph, as well as compliance of elements such as tubes within the plethysmograph and the resistance of openings through the plethysmograph. As will be appreciated, volumetric displacement of gas caused by an infant's breathing is not measured directly, rather, the air expelled from the interior of the plethysmograph through a resistive opening(s) through a wall of the plethysmograph is quantitatively measured. Although an infant's inspiration of gas will result in the expiration of air through the resistive opening in the plethysmograph (caused by a change in the volume of the infant), such inspiration will also tend to increase the pressure of air within the plethysmograph due to the resistivity of the opening, typically a fine mesh type structure, to gas flow. Similarly, expiration of gas from the infant's lungs will cause inspiration of air through the resistive opening accompanied by a decrease in the pressure within the plethysmograph. As a result, the quantity of gas expired/inspired by the infant's lungs and the quantity of air inspired/expired by the plethysmograph are not equal, thereby introducing another source of error which limits the accuracy and precision which may be achieved by known plethysmographs.
Additionally, leakage through various portions of the plethysmograph contribute to the introduction of error in calculations for determining flow rates and volumetric displacement. For example, leakage through the large seals which separate two halves of the plethysmographic chamber has been found to be a common source of error.
An additional deficiency of known plethysmographs is their inability to maintain a constant temperature environment for the infant. This disadvantage is also shared by other devices such as an incubator whose very purpose is to maintain a constant temperature environment for the infant. A constant temperature environment is critical to the survival of infants and, in particular, of premature infants.
Known incubators and/or plethysmographs generally attempt to maintain a constant temperature environment by either one of two methods, namely, by radiant heating or by convective heating. A typical radiant heater comprises a heat source such as a light source of appropriate wavelength radiating heat energy towards an exposed infant. A typical convective heater comprises a heating coil and, optionally, means for transporting air heated by such coil to the infant. Unfortunately, neither of these two methods has been found entirely satisfactory in maintaining a constant temperature environment for the infant.